In its simplest form, coffee roasting comprises heating a single bean to a prescribed temperature at which point chemical reactions occur that transform the bean into the desired state of pyrolysis. These reactions occur in the last part of the heating cycle. Thus, the residence time at the terminal temperature is crucial because a difference in a few seconds in heat-history can have a significant effect on the taste of the coffee.
The problem is that it is difficult to design a roaster that will roast several hundred pounds of beans at one time and to roast every bean evenly. Whether the process for heat transfer is from convection, conduction, radiation, or some combination thereof, the heat is absorbed in the first few layers of a bean bed. Therefore, it is desirable to establish some means for equalizing bean temperature throughout the heating cycle so that when the final roasting temperatures are approached, all of the beans will be close to the same temperature during the pyrolysis process.
The aforementioned problems have been overcome to a large degree by the Apparatus and Process for Conditioning Particulate Material disclosed and claimed in our co-pending U.S. patent application, Ser. No. 07/463,557, which was filed on Jan. 11, 1990, and which is incorporated herein in its entirety by reference.
In essence, a preferred embodiment of that invention comprises a chamber for receiving a charge of particulate material such as a charge of coffee beans. The chamber has a generally circular base and an upwardly extending divergent wall defining a segment of a cone with a central axis and closed bottom. The divergent chamber wall preferably forms an included angle with respect to a horizontal plane of between 40.degree. to 85.degree. and also defines a plurality of openings in a lower portion thereof. Means are provided for inducing a mass of heated fluid generally tangentially into the chamber to rotate the coffee beans about the central axis of the chamber and for maintaining the rotating material in a relatively densely packed or controlled state during the heating thereof. During the rotation of the coffee beans, the chamber is stationary, i.e., it does not rotate about its central axis so that there is relative movement between the rotating material and the stationary chamber. In addition, there is also vertical and radial movement of the coffee beans with respect to the chamber. This embodiment also results in horizontal shearing within the spinning bed since beans near the chamber bottom are rotating around the chamber at a higher angular velocity than the beans near the chamber top.
In a preferred form of the aforementioned apparatus, a second chamber is provided and disposed below the heating chamber for receiving the roasted beans and for rapidly cooling the beans in a similar manner.
Coffee roasters in accordance with our earlier invention uniformly roast batches of coffee very rapidly with an efficient use of energy. They also provide conditioning, cooling, heating and roasting apparatus which is relatively flexible, competitively priced, relatively simple in operation, free of complexity and easy to operate and maintain. In addition, such roasters occupy a relatively small area and can be rapidly converted to operate under different conditions in a job shop type of operation while fulfilling most of the requirements for food processing.
However, it has now been found that an improved apparatus, according to the present invention, provides all of the aforementioned advantages while adding improved flexibility. For example, the apparatus according to the present invention, can accommodate a greater range in charge variations, types of roast and greater control over the uniformity and quality of the roast.
The improved apparatus, according to the present invention, will accommodate relatively small to relatively large charges of beans and provides a more gentle bean action which results in fewer broken beans. The apparatus can also be programmed to maintain an optimal bean circulation during the roasting operation to compensate for the green to roasted bean expansion of almost 2:1. The apparatus also includes an improved bean spill which retains most of the coffee bean's tangential velocities while directing them to the bottom of the chamber. This reduces the likelihood of bean carry-over caused by slow moving beans in close proximity to the central updraft of the air leaving the bean bed and exiting the top of the chamber and is thought to be relatively easy to fabricate. The present design may also more readily accommodate a "well"-type temperature measurement which can be used to improve temperature control during the roasting operation.